Examples of microelectromechanical (MEMS) release structures include the micro-optical electromechanical system (MEOMS) membrane and cantilevered arm. Both of these structures are used in various optical applications. For example, they are fabricated to have a concave surface that is coated to be reflective and then paired with another mirror to form a tunable Fabry-Perot (FP) cavity/filter. They can also be used to define the end of a laser or interferometer cavity or shaped to function as a diffractive or refractive lens operating in transmission, for example.
The structures are typically produced by depositing a device layer over a sacrificial layer, which has been deposited on a support or handle layer. This sacrificial layer is subsequently completely or partially etched away or otherwise removed to produce the release structure in a release process. In some examples, the device layer is silicon or a silicon compound and the sacrificial layer can be polyimide or oxide, for example.
Typically, release structure deflection is achieved by applying a voltage between the release structure and a fixed electrode on the support structure. Electrostatic attraction moves the structure in the direction of the fixed electrode as a function of the applied voltage to control reflector separation or focal point location, for example.
One chronic problem associated with MEMS devices, in general, is stiction. The most common definition for stiction is the strong interfacial adhesion that is present between contacting crystalline microstructure surfaces. The term has also evolved to include sticking problems such as friction driven adhesion and humidity driven capillary forces on an oxide surface. Here, stiction is the unintentional adhesion in MEMS devices.
If deflected sufficiently to contact an adjoining surface, a release structure can adhere to, or snap-down on, that surface. This problem can be more intractable in the context of the optical release structures since anti-stiction coatings may not work well enough or may be incompatible with the required optical coatings, such as antireflective (AR) coatings or dielectric, highly reflecting (HR) coatings, for example. Moreover, these optical coatings are typically smooth because of the focus on optical performance, which smoothness typically increases the level of stiction forces in the event of contact since the magnitude of the stiction force is typically related to the contact area.
Stiction adhesion can be both a manufacturing yield problem and a performance problem after deployment. During manufacture, electrostatic charge build-up, caused by ambient atmospheric charge, electrostatic discharge (ESD) events during handling, or deposition processes, or capillary forces from wet-etch processes, for example, can lead to snap-down of the release structure. During operation, shock, excessive drive voltage, or system level ESD events can also cause snap-down.
Stiction adhesion is even more problematic in the case of electrostatically-driven MEMS microstructures. The electrostatic forces acting on the structure increase dramatically for a constant drive voltage as the structure passes through its stability point and travels across the electrostatic cavity toward the cavity""s stationary electrode. The structure can be further held against the contacting surface if the voltage is not immediately dissipated.
The present invention is directed to a microelectromechanical system. It is typically used in electrostatically operated micromechanical systems. Specifically, in order to prevent adhesion between a movable and another structure such as a stationary structure, a discharge system is activated upon pull-in of the movable structure to discharge the voltage across an electrostatic cavity to thereby prevent stiction adhesion of the movable structure to the stationary structure.
In general, according to one aspect, the invention features a micro-mechanical system. The system comprises a movable structure and a stationary structure. A cavity is provided across which an electrostatic voltage can be generated to pull-in the movable structure in a direction of the stationary structure. A discharge switch is activated by the pull-in of the movable structure to discharge the electrostatic voltage.
In general, according to another aspect, the invention features an electrostatically actuated optical system. This system comprises an optical element and an electrode defining an electrostatic cavity, across which an electrostatic voltage can be generated to drive movement of the optical element. A discharge switch is mechanically activated by pull-in of the optical element to discharge the electrostatic voltage.
In current implementations, the optical element includes a mirror structure that can be fabricated from a thin film (i.e., quarter wave thickness, typically) dielectric stack. Usually, the optical element is formed on a release structure. The release structure is currently a membrane. In the present embodiment, the discharge switch comprises a membrane conductor pad on the membrane that conducts a current upon activation of the discharge switch to discharge the electrostatic voltage. Currently, this pad is a metal. Preferably, there is also provided an opposed electrode conductor pad on the electrode that conducts the current upon activation. Upon pull-in, the membrane conductor pad contacts the electrode conductor pad.
In general, according to still another aspect, the invention features an electrostatically actuated optical system that comprises a support structure and a membrane structure that is separated from the support structure by an electrostatic cavity. Conductive stiction pads are provided between the support structure and the membrane structure that discharge a voltage across the electrostatic cavity in the case of pull-in of the membrane structure to the support structure.
Finally, according to still another aspect, the invention features a process for fabricating stiction features on an electrostatically driven optical membrane or other release structure. This process comprises releasing a device layer from a support structure to form a release structure and depositing support structure conductor pads on the support structure through the release structure. In this way, the release structure can functions as a shadow mask for the patterning of the support conductor pads, some implementation examples.
Depending on the embodiment, the discharge switch can be fabricated prior to or after the release of the release structure.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.